1. Technical Field
The present invention relates to telephone keyphone circuits and processes generally, and more particularly, to circuits and processes for displaying a caller identification for an abandoned call in an exchange system, as well as to circuits and processes for displaying a caller identification on a display device in case when a caller from an office line hangs up the call since there is no response after receiving and storing the caller identification when a ring is received in a key-phone system.
2. Background Art
Generally, a key phone system includes a plurality of telephone stations and a central key service unit (KSU) which interconnects the telephones with each other and to a selected one of a plurality of central office lines. The KSU provides control functions for the key-phone system and establishes communication paths between telephone stations. Each telephone station is provided with a hold button and a plurality of sets of central office lamps and buttons, each set corresponding on a one-to-one basis to the central office lines. The central office lamps allow the subscriber or user to visualize the status of each individual central office line. A central office lamp turns off while the central office line is idle, turns on while the central office line is in use, flashes slowly while the central office line is being held, and flashes rapidly while an incoming call is being received over the central office line. The central office buttons are available for selecting and seizing any of the central office lines. The KSU detects an operation of the central office buttons which may occur at any telephone set so as to connect the telephone station to the selected central office line. Examples of such a key telephone system are disclosed in U.S. Pat. No. 4,570,035 for Programmable Key Telephone System issued to Pinede et al., U.S. Pat. No. 4,682,353 for Key Telephone System issued to Inoue et al., and U.S. Pat. No. 5,513,256 for Key Telephone System issued to Komuro.
Depending upon its sophistication and complexity, a key phone system can provide a wide variety of system features. One notable feature is to display a message from a caller or a name of the caller when the subscriber is absent to response to the call as disclosed, for example, in Japanese Laid-Open No. 2-186894 for Key Telephone System issued to Hiyama, and Japanese Laid-Open No. 2-127846 for Key Telephone System issued to Okamoto. Caller identification information which is commonly available for conventional telephone system may also be adapted for key phone systems when the user is subscribed to the caller identification service. In a typical telephone system, the caller identification information is added to the ring signal at a telephone exchange system as a service. Thereafter, the telephone number of the caller is displayed on a telephone set of the subscriber after the telephone number information is decoded to the telephone number of the caller. As a result, the subscriber can identify the caller before the subscriber picks up a handset of the telephone system to answer the call. Exemplary telephone systems with caller identification are disclosed in U.S. Pat. No. 5,365,145 for Alphanumeric Caller Identification Telephone issued to Lim, U.S. Pat. No. 5,398,279 for Telephone Apparatus With Calling Line Identification issued to Frain, U.S. Pat. No. 5,446,785 for Telephone Terminal Equipment In Which Telephone Numbers Of Callers Are Displayed issued to Hirai, U.S. Pat. No. 5,452,346 for Telephone Apparatus With Caller Identification issued to Miyamoto, and U.S. Pat. No. 5,467,385 for Calling Number Display And Recording System issued to Reuben et al.
When the key phone system is subscribed to the caller identification service, however, the caller identification information is typically displayed on a display unit of a key phone terminal only while a ring is received in the key phone system. The caller identification information is not displayed on the display unit of the key phone terminal after the caller abandons his or her call for any reasons including when there is no answer from the subscriber. If the caller abandons or hangs up the call, as I have observed, the key phone system including a KSU and a plurality of extension line subscriber terminals is not equipped to allow the subscriber to verify the identity of the caller who quickly abandons his or her call primarily because of the complexity and difficulty of coordinating communications between the KSU and the extension line subscriber terminals.